A significant portion of electrical power produced throughout the world is produced in power plants that burn a fossil fuel (e.g., coal, oil, or gas). The burning of the fossil fuel provides heat that can be used to produce steam. This steam can then be used to drive a turbine and generator to produce electricity. Upon burning the fuel, a flue gas is also formed. In some cases, the flue gas itself is directly used to drive a turbine and generator to produce electricity. However, in either case, flue gas is formed as the fossil fuel is burned. The flue gas is ultimately removed from the power plant and discharged into the atmosphere by way of an exhaust stack.
The flue gas contains contaminants such as sulfur oxides (SOx), nitrogen oxides (NOx), carbon monoxide (CO) and particulates of soot or ash when coal is used as the primary fuel source. The discharge of all of these contaminants into the atmosphere is subject to federal and local regulations, which greatly restrict the levels of these flue gas components.
To meet the required levels of NOx emissions, many fossil fuel-fired electric generating units incorporate the use of selective catalytic reduction (SCR) technology. In this technology, ammonia or urea based reagents are typically injected in the presence of a catalytic converter to convert the NOx to nitrogen and oxygen. The catalytic converter is typically made of a substrate and a nitrogen oxide reduction catalyst. The nitrogen oxide reduction catalyst is the catalytic material that acts to convert the NOx to nitrogen.
When coal is used as a combustion fuel, fly ash, a solid residue, is also generated and mixed with the flue gas. Additional pollution control equipment, such as hoppers, electrostatic precipitators or a bag-house is used to capture the fly ash prior to release.
Depending upon the source and makeup of the coal being burned, the components of the fly ash produced vary considerably. Fly ash typically includes varying amounts of silica (silicon dioxide, SiO2) (both amorphous and crystalline), lime (calcium oxide, CaO), aluminium oxide (Al2O3), iron oxide (Fe2O3), and other types of calcium material (various calcium salts such as calcium carbonate and calcium sulfate).
Although separate equipment is used to remove the fly ash, over time the catalytic converters nevertheless become coated with a portion of the fly ash generated during combustion. Eventually, the catalytic converters become substantially reduced in their effectiveness, i.e., become deactivated, and have to be removed from service. Often, these fly ash-coated converters can be regenerated and put back in service.
Some of the methods of removing fly ash or fly ash materials from deactivated catalytic converters include treating the converters with water. Aqueous compositions that include acidic components are also used.
U.S. Patent Pub. No. 2009/0233786 discloses a method of regeneration of a SCR catalyst for use in a power plant facility burning fossil fuels, bio-based fuels, or a combination thereof, wherein poisons are removed from the catalyst. The method is suitable for use during neutralization using C1 to C8 carboxylic acids.
U.S. Patent Pub. No. 2006/0135347 discloses a method for the regeneration of deNOx catalysts with a reduced activity caused by the accumulation of phosphor and phosphorous compounds. The method is characterized in that the catalysts are treated with an essentially aqueous solution of water-soluble alkaline reacting alkaline earth salts, ammonium hydroxide, or alkaline reacting ammonium salts, or water-soluble organic amines with an approximate pK value ranging between 2.5 and 5.5 and that the excess alkali is neutralized by subsequent treatment with inorganic or organic acids.
U.S. Pat. No. 7,569,506 discloses a method for regenerating DeNOx catalysts having an increased SO2/SO3 conversion rate as a result of the accumulation of iron compounds, and is characterized in that the catalysts are treated with an essentially aqueous acid solution, preferably having a pH between 0.5 and 4, and with an addition of antioxidants.
U.S. Pat. No. 4,039,471 discloses a method for the rejuvenation of an automobile emission control catalyst poisoned with deposited compounds of lead and/or phosphorus is disclosed, said method comprising (a) initially extracting some of the poison compounds from the catalyst with a select aqueous ammonium or acetate salt (preferably ammonium acetate) solution; (b) exposing the catalyst to a reducing atmosphere at 300° C. to 700° C., and finally (c) removing at least a portion of the poison compounds by a second extraction with a select aqueous ammonium or acetate salt solution. Catalysts rejuvenated by the method include those comprising nickel, palladium, rhodium, platinum, or combinations thereof as active components on bases of alumina or aluminum borate.
Substrates or catalytic converters that contain calcium material deposited thereon, particularly calcium-containing fly ash material, have been found particularly difficult to clean, and ultimately, rejuvenate. Additional methods of treating substrates and rejuvenating catalytic converters are highly desired.